Geophysics improves our understanding of subsurface geological features. During the middle of the 20th century, ground penetrating radar (GPR) and gravity methods were rapidly developed to investigate subsurface problems as described in Reynolds, J. M, An introduction to applied and environmental geophysics, Wiley, 1997. However, the last three decades have featured the most important achievements in application of both the GPR and gravity methods in several fields, such as archaeology, engineering and environmental science. The use of GPR has increased in detecting and identifying many subsurface features, such as mineshafts, pipelines, ore lodes, cavities, groundwater, and buried rock valleys as described in Chamberlain, A. T., Sellers, W., Proctor, C. and Coard, R., “Cave detection in limestone using ground penetrating radar”, Journal of archaeological science, vol. 27, p. 957-964, 2000.
Subsurface cavities are hazardous due to their susceptibility to ground surface subsidence, which can cause great losses to the population that occupies the land above them as described in Benson, A. K., “Applications of ground penetrating radar in assessing some geological hazards: examples of groundwater contamination, faults, cavities”, Journal of Applied Geophysics, vol. 33, p. 177-193, 1995 incorporated herein by reference in its entirety. Cavities are formed either naturally through the dissolution of limestones, dolomites, and evaporites or by human action, such as the construction of tunnels and tombs as described in Chalikakis, K., Plagnes, V., Guerin, R., Valois, R. and Bosch, F. P., “Contribution of geophysical methods to karst-system exploration: an overview”, Hydrogeology journal, vol. 19, p. 1169-1180, 2011 incorporated herein by reference in its entirety. Cavities may be filled with air, water, sediments, or a combination of materials.
There are many geophysical techniques that are appropriate for detecting and/or defining cavities features in the subsurface. For instance, microgravity as described in: Butler, D. K., “Interval gravity-gradient determination concepts” Geophysics, 49, p. 828-832 1984, Bishop, I., Styles, P., Emsley, S. J., and Ferguson, N. S., “The detection of cavities using the microgravity technique: case histories from mining and karstic environments”, Modern geophysics in engineering geology, geological society, Engineering geology special publication, vol. 12, p. 153-166,1997, Styles, P., McGrath, R., Thomas, E. and Cassidy, N. J., “The use of microgravity for cavity characterization in karstic terrains”, quarterly journal of engineering geology and hydrogeology, vol. 38, p. 155-169, 2005 and Panisova, J., and Pasteka, R., “The use of microgravity technique in archaeology: A case study from the St. Nicolas church in Pukanec, Slovakia” contributions to geophysics and geodesy vol. 39, p. 237-254, 2009, ground penetrating radar as described in Chamberlain, A. T., Sellers, W., Proctor, C. and Coard, R. “cave detection in Limestone using ground penetrating radar”, Journal of Archaeological science, vol. 27, p. 957-964, 2000, electrical resistivity tomography as described in Gambetta, M., Armadillo, E., Carmisciano, C., Stefanelli, P., Cochic, L. and Tontini, F. C., “Determining geophysical properties of a near surface cave through integrated microgravity vertical gradient and electrical resistivity tomography measurements”, Journal of cave and karst studies, vol. 73, p. 11-15, 2011, Putiska, R., Nikilaj, M., Dostal, I., and Kusnirak, D., “Determination of cavities using electrical resistivity tomography”, contributions to geophysics and geodesy, vol. 42, p. 201-211, 2012 and Metwaly, M. and Alfouzan, F, “Application of 2-D geoelectrical resistivity tomography for subsurface cavity detection in the eastern part of Saudi Arabia”, Geoscience Frontiers, vol. 4, p. 469-476, 2013, seismic refraction and reflection as described in Pernod, P., Piwakowski, B., Delannoy, B. and Tricot, J. C., “Detection of shallow underground cavities by seismic methods: physical modelling approach”, Acoustical Imaging, vol. 17, p. 705-713,1989 and Fiore, V. Di., Angelino, A., Passaro, s. and Bonanno, A., “High resolution seismic reflection methods to detect near surface cavities: a case study in the Neapolitan area, Italy”, Journal of cave and Karst studies, vol. 75, p. 51-59, 2011, and transient electromagnetic methods (TEM) as described in Xue, G., Song, J., Xian, Y., “Detecting shallow caverns in China using TEM”, The leading edge, vol. 23, 2004. The strength of the GPR and gravity methods to detect subsurface cavities is mainly due to their ability and relative ease of detecting the contrast in physical properties between the surrounding soil or rock and the materials that fill these cavities.
The GPR and gravity methods are nondestructive geophysical techniques that measure differences in the physical properties of the subsurface materials, such as the dielectric permittivity for the GPR method and the density contrast for the gravity method. The success of the GPR and gravity methods depends on the different subsurface materials having different dielectric permittivities and bulk densities, which produce variations in the measured electromagnetic wave velocity (due to variations in matrix composition and water content) and gravity field.
The literature records many studies describing qualitative investigations of cavities via utilizing several geophysical tools. A few typical negative gravity anomalies related to shallow cavities in large caverns in Iraq was described in Colley, G. C., “the detection of caves by gravity measurements”, Geophysical prospecting, vol. 11, p. 1-9,1963. The main components of subsurface cavity complex systems could be detected with microgravity measurements was shown in Butler D. K., “Interval gravity-gradient determination concepts”, Geophysics, vol. 49, p. 828-832, 1984. The initial physical geological models was developed by computing the second and third derivatives of the gravity potential. In addition, the microgravimetric and gravity gradient techniques are used as effective tools to the delineation and detection of shallow subsurface tunnels and cavities as described in Butler, D. K., “Microgravimetric and gravity gradient techniques for detection of subsurface cavities,” Geophysics, vol. 49, p. 1084-1096, 1984.
Promising results from using GPR for detecting cavities in limestone in an area of archaeologically important karst topography were presented in Chamberlain, A. T., Sellers, W., Proctor, C. and Coard, R., “Cave detection in limestone using ground penetrating radar”, Journal of archaeological science, vol. 27, p. 957-964, 2000. The GPR and microgravimetric methods as tools for detecting and characterizing shallow subsurface karstic features in terms of shape, size and depth were described in Beres, M., Luetscher, M., and Olivier, R., Integration of ground penetrating radar and microgravimetric methods to map sallow caves, Journal of applied geophysics, vol. 46, pp. 249-262, 2001. GPR to delineate near-surface fractures within Dammam Dome was described in Al-Shuhail, A. A., Hariri, M. M. and Makkawi, M. H., “Using ground penetrating radar to delineate fractures in the Rus Formation, Dammam Dome, Eastern Saudi Arabia”, International geology review, vol. 46, p. 91-96, 2004.
A new idea using GPR and seismic data quantitatively for estimating water saturation and porosity in subsoil using a shared earth model is described in Ghose, R., and Slob, E. C., “Quantitative integration of seismic and GPR reflections to derive unique estimates for water saturation and porosity in subsoil”, Geophysical research letters, vol. 33, 2006 incorporated herein by reference in its entirety. Local density variations caused by a near surface void results in negative anomalies detectable using microgravity techniques as described in Panisova, J., and Pasteka, R., “The use of microgravity technique in archaeology: A case study from the St. Nicolas church in Pukanec, Slovakia” contributions to geophysics and geodesy vol. 39, p. 237-254, 2009.
Vertical gradient microgravity and electrical resistivity tomography can be used to give high-resolution images of underground voids as described in Gambetta, M., Armadillo, E., Carmisciano, C., Stefanelli, P., Cochic, L. and Tontini, F. C., “Determining geophysical properties of a near surface cave through integrated microgravity vertical gradient and electrical resistivity tomography measurements”, Journal of cave and karst studies, vol. 73, p. 11-15, 2011.
A new approach for modeling subsurface cavities characterized by typical geometries such as sphere, vertical and horizontal cylinder by using a linear neuro-fuzzy microgravity technique was described in Hajian, A., Zomorrodian, H., Styles, P., Greco, F. and Lucas, C., “Depth estimation of cavities from microgravity data using a new approach: the local linear model tree”, near surface geophysics, vol. 10, p. 221-234, 2012 incorporated herein by reference in its entirety.
Shallow cavities in tuff layer characterized by a high acoustic impedance contrast can be easily detected with a high resolution P-wave seismic reflection technique as described in Fiore, V. Di., Angelino, A., Passaro, S. and Bonanno, A., “High resolution seismic reflection methods to detect near surface cavities: a case study in the Neapolitan area, Italy”, Journal of cave and Karst studies, vol. 75, p. 51-59, 2011.
2D electrical resistivity survey was used in a newly urbanized area in Alhassa of Saudi Arabia to delineate different cavities as described in Metwaly, M and AlFouzan, F., Application of 2-D geoelectrical resistivity tomography for subsurface cavity detection in the eastern part of Saudi Arabia, Geoscience Frontiers, vol. 4, p. 469-476, 2013.
The presence of cavities features in the subsurface rock and their potential collapse pose an acute geohazard and constitute a risk to developed land. The delineation of subsurface cavities and the determination of porosity and water saturation parameters of cavity filling materials are important in many geotechnical applications. Drilling is a most common method of site investigation in an attempt to detect and characterize physical parameters of subsurface cavities such as porosity and water saturation but it is expensive. In order to overcome this issue, non-invasive and cost-effective geophysical methods can be used for cavity detection. Due to the importance of estimating porosity and water saturation to civil engineers in designing better structures, as recognized by the present inventor, a joint inversion approach of GPR and gravity data is presented for calculating these two parameters.
To date there have not been attempts to combine GPR and gravity petrophysical methods to estimate porosity and water saturation of cavity filling materials. Previous GPR and gravity studies have been applied as integration on subsurface cavity detection using a qualitative approach.
The foregoing “background” description is for the purpose of generally presenting the context of the disclosure. Work of the inventor, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention. The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.